The present invention relates to the art of control systems for electrical motors. More particularly, the present invention relates to the art of control systems for DC motors using all-digital phase-locked loops.
Control systems for electrical motors have historically been analog devices because the electrical motor itself is an analog device. However, with the increased use of digital circuitry to control a variety of applications, electrical motor control is also evolving toward digital control. Electrical motors, and in particular DC servo motors, find a variety of applications from use in computer disk drives, to use in robotic machinery and mechanical printing devices.
Color inkjet printers are one form of mechanical printing devices that have recently emerged as the leader in the desktop and small business market. Today, color inkjets can provide near laser quality printing at a fraction of the cost of laser printers. In the art of black-and-white printing, and in the applications of non-networked desktop offices and home use computers, inkjet printers are finding wide acceptance over low-end laser printers. This has created a large demand in the inkjet printer market for low-cost, high quality printers.
With this increase in demand and competition, important factors include improved quality and lower cost. In inkjet printers, a microprocessor typically controls the printing process. All of the digital circuitry needed to support the microprocessor is placed into an Application Specific Integrated Circuit (xe2x80x9cASICxe2x80x9d). Advances in both microprocessor and integrated circuit technology have been major areas of cost reduction in the past. By replacing the processing burden of the microprocessor with digital circuitry in the ASIC, microprocessor bandwidth may be reduced, thereby facilitating the use of less expensive microprocessors or even the eventual removal of the microprocessor.
Inkjet printers produce output by spraying drops of ink onto a page. Some ink-jet printers, instead of projecting the ink electrostatically or with very tiny pumps, vaporize the ink within a heated capillary. The pressure of expanding gas then jets the expanding vapor onto the page. The inkjet heads, which are used to deposit the ink onto the paper, are attached to a carriage, which houses the ink reservoirs. In the printing process, this carriage must be moved across the page at a constant rate in order to deposit the ink onto the paper in a uniform manner. The carriage is traditionally belt driven by a DC servo motor.
Conventionally, the velocity of the carriage is controlled by a microprocessor. The load of the carriage changes with position, and the mass of the ink left in the reservoir changes with time, thus making the system time-varying. The DC servo motor with belt drive connection to the carriage is a highly non-linear system. Precise velocity control of the non-linear time-varying carriage system will require a control scheme with a fairly high sample rate. Thus, implementing this scheme of precise velocity control in a microprocessor traditionally demands a majority of the microprocessor bandwidth during print moves.
In the last 20 years, with the reduction in cost of digital systems, the use of phase-locked loop (xe2x80x9cPLLxe2x80x9d) motor control systems has increased. Although PLL systems were originally designed for applications involving synchronization of signals in communication systems, A. W. Moore showed in xe2x80x9cPhase-Locked Loops for Motor-Speed Control,xe2x80x9d IEEE Spectrum. April 1973, p. 61-67, that PLL systems can be applied to motor speed control. In fact, it has been shown that speed control with an accuracy approaching 0.002% is possible.
Originally, PLLs were designed and implemented exclusively in analog circuitry. In 1960, Westlake, set forth in xe2x80x9cDigital Phase Control Techniques,xe2x80x9d IRE Transactions on Communications Systems v.CS-8, December 1960, p. 237-246, that a digital Voltage Controlled Oscillator (xe2x80x9cVCOxe2x80x9d) could be incorporated into a PLL. Various developments continued in the 1960s in an effort to replace the analog PLL components with digital circuitry. In most work, however, the PLL system remained a hybrid mix of digital and analog components, called the Digital Phase-Locked Loop (xe2x80x9cDPLLxe2x80x9d). Accordingly, hereinbelow, the term xe2x80x9cdigital phase-locked loopxe2x80x9d shall refer to a hybrid mix of digital and analog components. In 1967, Drogin, set forth in xe2x80x9cSteering on Course to Safer Air Travel,xe2x80x9d Electron Nov. 27, 1967, p. 95-102, that an entire PLL could be implemented with digital components for use in a communication control system, and thereby created the first All-Digital Phase-Locked Loop (xe2x80x9cADPLLxe2x80x9d). Thus, hereinbelow, the term xe2x80x9call-digital phase locked loopxe2x80x9d shall refer to a PLL, which is implemented entirely with digital components.
All PLL motor control systems to date have hence been either all analog or DPLL configurations. Today, systems that were implemented with DPLLs conventionally retain an analog loop filter. This is convenient for most motor control systems because the input to the motor is a variable analog voltage. However, in order to easily realize a PLL control system in an application specific integrated circuit (xe2x80x9cASICxe2x80x9d), the loop filter is preferably a digital device. Analog loop filters are less precise and require analog components such as resistors and capacitors, which vary from vendor to vendor.
One form of digital phase-locked loop filter for use in motor control is set forth by Shigemori in U.S. Pat. No. 4,816,722. Shigemori uses a digital loop filter in the form of an up-down counter. The count from the up-down counter is clocked into an adder, and an overflow signal is then transmitted through a frequency divider to produce a feedback output signal. The digital loop filter in the form of an up-down counter suffers from the disadvantages of locking in a state when the phase error is zero.
It is therefore an object of the present invention to present a solution to the above problems in the art and to present a novel all-digital phase-locked loop system to control the velocity of a DC motor, and in particular an inkjet print head transport system. The ADPLL system is preferably implemented in an ASIC using standard digital devices. The system preferably provides for printer steady-state error, overshoot, and settling time.
It is a further object of the present invention to provide an inkjet print head transport system that provides less than xc2x15% steady error and less than 20% overshoot. Also, the carrier preferably accelerates to the desired print speed at a rate of 0.8 g, where g is the acceleration due to gravity. A design routine is presented herein in accordance with the above.
It is a further object of the invention to provide velocity control of an inkjet print head by utilizing phase-locked loops, digital phase-locked loops, and all-digital phase-locked loops. The PLLs are used to control DC servo motors. Components of the PLLs preferably include a phase detector and a loop filter.
It is a further object of the present invention to provide a DPLL to control velocity of an inkjet print head carriage system. A mathematical model of the DPLL system is utilized to provide a non-linear analysis of a phase-frequency detector. Also, simulated results of a closed-loop system are given for various print speeds.
It is still a further object of the invention to provide a method for converting an analog loop filter into a digital loop filter for use in an all-digital phase-locked loop. Conversion of the analog loop filter provides for the implementation of the all-digital phase-locked loop in an application specific integrated circuit (xe2x80x9cASICxe2x80x9d). Conversion of the analog loop filter into a digital loop filter requires simulated and experimental results of a controller.
Objects of the present invention are achieved by a control system for controlling movement of a DC motor, including a movement detector to detect movement of a DC motor and output a corresponding feedback signal; a digital phase detector to compare phase of the feedback signal from the movement detector with phase of a reference signal and output a comparison signal, wherein the digital phase detector follows a describing function to model non-linear components of the reference signal; and a digital loop filter to filter noise from the comparison signal for control of the DC motor.
Further objects of the present invention are achieved by providing the digital phase detector and the digital loop filter within an application specific integrated circuit, and by using the phase frequency detector to model non-linear components of a reference signal.
Even further objects of the invention are achieved by a control system for controlling velocity of an inkjet print head in response to a reference signal, including a velocity detector to detect velocity of an inkjet print head and output a corresponding feedback signal; a digital phase frequency detector to compare phase of the feedback signal from the velocity detector with the reference signal and output a corresponding comparison signal; a digital loop filter to filter noise from the comparison signal; and a motor to control velocity of the inkjet print head in response to the filtered comparison signal.
Moreover, objects of the present invention are achieved by a method for controlling velocity of an inkjet print head in response to a reference signal, including the steps of: detecting a velocity of an inkjet print head and outputting a corresponding feedback signal; comparing phase of the feedback signal with phase of the reference signal using a digital phase frequency detector and outputting a corresponding comparison signal; filtering noise from the comparison signal; and controlling velocity of the inkjet print head in response to the filtered comparison signal.
Still further objects of the present invention are achieved by a control system for controlling velocity of an inkjet print head in a system including a velocity detector to detect inkjet print head velocity and output a corresponding feedback signal, a phase detector to compare phase of the feedback signal with phase of a reference signal and output a corresponding comparison signal, a digital loop filter to filter noise from the comparison signal, and a motor to control velocity of the inkjet print head in response to the filtered comparison signal, creation of the control system including: modeling the phase detector in a closed loop, phase locked loop configuration with a describing function; and dampening the frequency response of the describing function by anticipating a step response of the motor during initial movement.